Sarcopenia and weakness are inevitable consequences of normal aging that reduce function, predispose to falls and fractures and are thus associated with significant morbidity and healthcare costs. A growing body of evidence implicates weakness and loss of muscle power as important contributors to falls in the elderly; proposed mechanisms include reduced neural drive, loss of fast-twitch fibers, infiltration of muscle by adipose or other tissue types, dysfunction of the myofibrillar apparatus, or impairments in excitation contraction (E-C) coupling. Ineffective E-C coupling is thought to be an important contributor to age-related muscle weakness. EC-Coupling links depolarization of the sarcolemma to a rise in cytosolic calcium concentrations which in turn causes shortening of actin-myosin fibrils and muscle contraction. This application focuses on roles of the adaptor protein Numb in regulation of cytosolic calcium transients required for E-C coupling. We propose that reductions in Numb expression impair E-C coupling and contribute to aging-associated weakness. Numb has critical roles in cell fate determination, asymmetric cell division and vesicular trafficking. In skeletal muscle, Numb is required for satellite cell proliferation and enhances myogenic differentiation potential. No studies have investigated roles of Numb in skeletal muscle E-C coupling or investigated whether reduced Numb expression contributes to weakness during aging. Our preliminary studies show that Numb is present in skeletal muscle fibers where it localizes near DHPR. Expression of Numb was reduced in muscles from 20- month old mice. A knockout of Numb and NumbL (a closely related protein with overlapping functions) in skeletal muscle fibers reduced muscle strength. Studies of primary cultures of mouse myoblasts revealed that a knockdown of Numb reduced the caffeine-induced rise in intracellular calcium concentration. These observations provide compelling evidence for a role of Numb in muscle force production, localize Numb to the triad, and implicate Numb in regulating calcium transients in skeletal muscle fibers. The findings also implicate reduced Numb expression as a causal determinant of impaired E-C coupling of aging. We hypothesize that: 1) diminished specific force generation in our Numb/NumbL double-knockouts is due to reduced Numb expression, 2) Numb is required for proper regulation of EC-coupling because 3) Numb regulates release by RyR1 of calcium stored in the SR and absence of Numb depletes SR calcium stores and 4) that age-related decreases in Numb expression are a cause of weakness. To test these hypotheses we will: Aim 2, Determine the cellular and molecular basis for muscle weakness resulting from Numb/NumbL knockdowns; Aim 2, Determine the role of reduced Numb expression in aging-related declines in force production.